The youngest geologic units on Guam are Holocene deposits of limestone, beach sand, and alluvium. The Holocene limestone is known as Merizo Limestone and is the youngest rock unit on Guam, with its available radiocarbon ages within the interval 4500-2750 BP (Dickinson, 2000). This emergent coral-algal reef locally caps Mariana reef rocks and basalt units at sea level and is exposed only in the intertidal and sea spray zones. Retaining high primary porosity, it generally shows no recrystallization of primary minerals.

Beach deposits in northern Guam occur in places where the cliffs are retreated or form embayments. The sand is mostly biogenic, consisting of locally-derived calcareous material: fragments of coral, coralline algae, mollusks, Halimeda algae, echinoid spines, and tests of benthic foraminifera. The thickness of modern beach deposits is estimated to be several meters. Classic beachrock deposits are common. They consist of stacked layers of cemented local beach sand.

Alluvial deposits are uncommon in northern Guam and are absent from most of the plateau surface due to lack of surface flow. They are only significant in the southern part of the plateau, where the land comprises of argillaceous limestone and there is a number of active and dry river valleys.